The Preparation and Properties of CVD-Silicon Tubes and Boats for Semiconductor Device Technology

Abstract

The publication costs o~ this article l~ave been assisted by the Dow C o m i n g Corporation.At present quartz is the most important material for producing tubes and boats for diffusion and oxidation processes in silicon device technology. The advantage of quartz compared to other materials is its combination of purity, resistance to mechanical and temperature changes at high temperature, and good workability. But there are limits for the application of quartz. At temperatures exceeding 1200~ it becomes increasingly soft and devitrifies, especially where it contacts other materials. In this state, quartz tubes slowly sag and flatten, and also become more permeable to gases and trace impurities. High ter~peratures are, however, necessary to accelerate the different diffusion and oxidation processes.Quartz can be obtained in a relatively pure quality. However, even these pure grades contain undesirably high levels of some metal impurities such as sodium, which must be excluded from various oxidation processes.One of the most disturbing effects in silicon device processing is the generation of clusters of dislocations at contact points between the quartz boat and a silicon wafer during high temperature processing, partly caused by mechanical stress at these points of the wafer. This phenomenon is shown in Fig. 1 by a wafer that had been dislocation-free before processing. The mechanical stress that produced these dislocations was due to the difference in thermal expansion coefficients between the silicon wafers and the quartz boat.The use of silicon tubes prepared by chemical vapor deposition (CVD) eliminates most of the difficulties encountered with quartz tubes. Polycrystalline silicon does not soften like quartz at temperatures approaching 1400~ its resistance to cracking caused by thermal shock is excellent; its purity level is very high; and its coefficient of expansion is nearly identical with that of the silicon wafers.Si-tubes and boats can also be made from monocrystalline or polycrystalline bulk silicon by coring, sawing, and grinding methods. This is, however, an expensive and difficult process due to the somewhat brittle material. Preparation of Silicon TubesPolycrystalline silicon tubes and other shaped hollow bodies are produced by the same basic CVD method used to deposit semiconductor-grade silicon rods: the hydrogen reduction of pure trichlorosilane. In the case of hollow bodies, however, it is necessary * Electrochemical Society Active M e m b e r . K e y w o r d s : s e m i c o n d u c t o r t u b e s a n d boats, silicon tubes and boats, quartz tubes and boats, vapor d e p o s i t i o n , t r i c h l o r o s i l a n e reduction, silicon deposition.Fig. 1. Clusters of dislocations generated on a silicon wafer at its two contact points with a quartz boat used during processing.to deposit silicon on a substrate such as pure graphite (1,2), the outside dimensions of which become the inside dimensions of the silicon form being grown. The over-all chemical reaction taking place at temperatures exceedin...

Semiconductor measurement technology

“…The resulting polycrystalline silicon has a sodium content of about 10 ppb by weight [29] or about 1000 times less sodium than occurs in transparent fused silica oxidation tubes . The low sodium content of polysilicon makes it an excellent wall material to contain oxidation atmospheres.…”

Thermodynamic Considerations in the Use of Polysilicon Oxidation Tubes for Clean SiO2 Film Preparation

“…When recently installed, new silicon tubes have a low sodium content and thus constitute an excellent material to enclose the oxidation atmosphere. This is due to the tube fabrication process (chemical vapor deposition) employing highly purified (low alkali) materials (28). Preparation of clean low mobile ion content thermal oxide films by wet oxidation of silicon wafers is in principle possible if silicon oxidation tubes are used, provided that the tube environment is clean enough to avoid inclusion of ambient contamination in the tube.…”

“…This is caused by the low sodium content in silicon tubes fabricated by chemical vapor deposition through hydrogen reduction of trichlorosilane. This process produces high purity polysilicon with only 10 ppb sodium contamination, 1000 times less sodium content than in fused silica tubes (28). However, due to external contamination sources (i.e., sodium from refractories and room ambient), periodic in situ cleaning of the tube is necessary to maintain appropriate conditions in the oxidation atmosphere.…”

Development of Sodium Contamination in Semiconductor Oxidation Atmospheres at 1000°C

“…or (ii) use of sodium-leaching agents to deplete a region just under the inner wall surface prior to oxidations. The first approach is based on the well-known fact that polycrystalline silicon tubes can be produced with 1000 times less sodium content than found in silica tubes (17). Empirical results indicate that oxide films grown on silicon using recently installed silicon oxidation tubes produce cleaner films (lower sodium content) than similar oxides prepared in fused silica tubes.…”